1. Technical Field
The invention relates to steel sheet forming processes, more particularly to a technique of bending high-strength steels.
2. Related Art
Because metals have good strength and flexibility, they are usually used as a structural element requiring strength. When a force applied on a unit area of metal exceeds its tensile strength, the metal will not split like ceramic materials, but will be deformed. That is to say, metals have a property of plasticity, i.e. plastic deformation. Usually, a power press machine is used for deforming a metal. There is a concave die and a convex die in a power press machine. By means of stroke, the metal material being placed between the two dies can be deformed as the shape formed by the dies.
FIG. 1 illustrates a conventional technique of forming a high-strength steel (HSS) sheet into a C-shape. The HSS 10 is placed between a concave die 20 and a convex die 22. During the pressing process, two depressors 24 are disposed beside the convex die 22 and used for clamping two opposite sides 12 of the sheet 10. As shown in FIG. 2, however, after the pressing action, the sides 12 of the sheet 10 tend to become an undesired warp or curl (i.e. “recoil”) because of its inner stress against the pressing force. Thus, the quality of HSS sheet forming will be adversely affected. As shown in FIG. 2, the shape 12 depicted by broken lines is the desired shape of sheet, but a really resultant shape is like the shape 12′.
Moreover, an additional post-production is required because the desired shape can not be accomplished at a time. Therefore, the conventional forming process with post-production includes the steps of : a)forming; b)cutting; c)trimming and d)re-trimming or, a)loading; b)forming; c)re-forming; d)trimming and e)re-trimming. Any of the above processes is lengthy and tardy. The most important point is that pressing veins or scratches will be left on the sheet after these steps. Quality of the finished products will be considerably low unless a finally additional polishing step is performed.
For example, in order to avoid warps or curls, and to increase accuracy of the products formed, the steps of forming and trimming must be repeatedly performed. It is very uneconomic. Furthermore, the forming process with multiple forming and trimming steps may also reduce lifetime of the dies. Meanwhile, the formed products will be hardened. It is disadvantageous to the latter process.